The invention relates to open or transparant communications systems and more particularly, but not exclusively, to private communications systems providing for digital transmission of voice and data in the form of communication data blocks also called packets.
The basic ideas underlying modern packet switching technology are well known and will not be repeated here, references are publications such as "A digital communication network for computers giving rapid response at remote terminals" from D.W. Davies at al in Proc. ACM Symposium on operating system principles, Gatlinburg 1967 and more recently "Data Processing Open Systems Interconnection--Basic Reference Model" from the International Standardisation Organisation, published in Computer Networks 5 (1981) pages81-118.
An advantage of packet switching is that each communication data block is an autonomous entity. If the communication network has several alternative routes, the switching of the communication data blocks leads to optimum use of the network capability. To be able to rearrange received communication blocks from alternative routes, having different transmission times, in the right sequence each of the communication data blocks is provided with a communication header containing a specific code word with cyclic numbering. A loss of a communication data block can easily be detected by comparing the code word number of a received communication data block with the code word number of the immediately beforehand received communication data block as e.g. known from U.S. Pat. No. 4,398,290.
Missing communication data blocks are restored by means of known error recovery procedures.
To prevent that, after a succesfull correction of the flow of communication data blocks, the original missing communication data block and or missing information data blocks used by the error recovery procedure could be confused with new blocks and/or information data blocks, the range of the cycle of the cyclic numbering of the code word had to be in correspondence with the warranted maximum lifetime of the communication data block and/or information data block in the network.
This has not only the disadvantage that the code words have a large number of bits in the communication header of the communication data blocks and thereby reducing the maximum transmission efficiency of the network but has especially the disadvantage that ongoing interconnection of existing networks and the introduction in the network of satellite links, influencing the maximum lifetime, are not possible without enlarging the range of the cyclic numbering, which cannot easily be realized.